What it is
RSUs vest on a schedule and become taxable income at vesting. Options vest on a schedule, and post-termination exercise windows have been lengthened by law for qualified options. Termination usually stops future vesting, and some plans accelerate on retirement, death, or a change of control.
What to do about it: Read the post-termination exercise window, the forfeiture clause, and the change-of-control trigger before your last day. The window sets the tax year.
Plain-language description of US practice, not legal, tax, or financial advice. The paired guide carries the citations, dates, and thresholds; the state pages carry the local windows.